Critical Distance Default Values for Structural Steels and a Simple Formulation to Estimate the Apparent Fracture Toughness in U-Notched Conditions

Abstract

The structural integrity assessment of components containing notch-type defects has been the subject of extensive research in the last few decades. The assumption that notches behave as cracks is generally too conservative, making it necessary to develop assessment methodologies that consider the specific nature of notches, providing accurate safe predictions of failure loads or defect sizes. Among the different theories or models that have been developed to address this issue the Theory of Critical Distances (TCD) is one of the most widely applied and extended. This theory is actually a group of methodologies that have in common the use of the material toughness and a length parameter that depends on the material (the critical distance; L). This length parameter requires calibration in those situations where there is a certain non-linear behavior on the micro or the macro scale. This calibration process constitutes the main practical barrier for an extensive use of the TCD in structural steels. The main purpose of this paper is to provide, through a set of proposed default values, a simple methodology to accurately estimate both the critical distance of structural steels and the corresponding apparent fracture toughness predictions derived from the TCD.

A Short Review on Fracture Mechanisms of Mechanical Components Operated under Industrial Process Conditions: Fractographic Analysis and Selected Prevention Strategies

“…Various fracture mechanics models are used to analyze the fracture toughness of notched components; the most known ones are the theory of critical distances (TCD) [44,45], strain energy density (SED) [46], and cohesive zone model (CZM) [47]. The rigorous elaboration of the above mentioned fracture mechanics models is out of the scope of the present work; rather a short highlight of the TCD is worth mentioning.…”

“…In case of full LEFM conditions, σ 0 coincides with the ultimate tensile strength (σ UTS ). A relevant study of the fracture toughness of U-notched components using the TCD is given in [45].…”

“…The parameter, L, presented as the length constant, can be calculated in various different ways [44,45]. Particularly, a number of researchers have proposed that failure can be predicted by modifying the critical stress idea so that the stress to be used is not the maximum stress (at the notch root), but the stress at a point located at a certain distance from the notch.…”

Brittle Failure of Nanoscale Notched Silicon Cantilevers: A Finite Fracture Mechanics Approach

“…Generally speaking, for all the methods based on a length parameter, the latter can be related to Irwin's length l ch = (K Ic /σ u ) 2 , with l ch ≈ 5.18 nm in the present work. Recalling the synthesis by means of the TCD presented in [30,36], it could be asserted that all the critical distances are proportional to [53]:…”

Finite Fracture Mechanics Assessment in Moderate and Large Scale Yielding Regimes

“…Following the same approach and using the same stress criteria, analogous considerations were put forward after tensile testing U-notched samples made of Al 7075-T651 [9]. Indeed a slightly different approach was proposed in [10], where it was suggested, instead of working on the tensile strength, to calibrate the critical length when dealing with structural steels.…”